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2GLSS P.H. Regan 3 Is the Universe Infinite ? Olber’s Paradox (RJS p534, CO p1222) Q. Why is the sky dark at night ? If the universe was infinitely large and old, you would see a star in your line of sight in all directions, the night sky should be bright! This is evidently NOT the case, ‘Olber’s Paradox’. Olber’s ‘solution’, space not transparent BUT this wouldn’t matter as any interstellar dust would be heated to the same temp. as stellar surface and thus glow the same colour. Also proposed was that the recession velocity moved the light out of visible wavelengths (‘redshifted’), BUT the shift is not large enough. A. Light has a finite speed (c=3x10 8 ms -1 ) and the light from the furthest stars has not reached the earth yet (solution proposed by Lord Kelvin and Edgar Allen Poe!) Thus the observable universe is finite in size (and age).

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2GLSS P.H. Regan 5 Nuclei: Stellar Fuel. (CO p349) Elements up to Fe (Z=26) can be formed by nuclear fusion, which keeps the star ‘burning’. H (Z=1), burns to He (Z=2), which burns to Carbon (Z=6). Then Carbon, Oxygen (Z=8) and Silicon (Z=14) burning are all allowed in large (heavy, hot and dense) stars. For Z>26 fusion is no longer energetically favourable, but heavier elements can be formed by neutron capture followed by  - decay. (via the ‘slow’ (s) or ‘rapid’-neutron processes).

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2GLSS P.H. Regan 8 Elemental Abundances (CO p526, RJS p109, 307) Note x-rays from solar photosphere show evidence of elements up to U (Z=92) in the sun. Large stars live for only around years, very short compared to universal time scales. After this, large stars collapse and expel much of their reaction products into space for future star formation.

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2GLSS P.H. Regan 11 Emission and Absorption Lines Star light (black body) Interstellar medium ( diffuse gas and dust) Absorption lines (observation) Emission lines (observation) Most (approx 70%) of the Inter-Stellar Medium (ISM) is made up of hydrogen, in either atomic or molecular form. For Hydrogen ATOMS the principal lines come from the Lyman, Balmer and Paschen Series

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2GLSS P.H. Regan 12 A hot, dense gas (or solid object) produces a continuous spectrum with no dark spectral lines. (Black-body spectrum) A hot, diffuse gas produces bright emission lines when an electron makes a transition from a higher excited state to a lower one. The wavelength of the emitted photon can be calculated from the energy difference between the initial and final levels. A cool diffuse gas in front of a black-body source produces dark, absorption lines when an electron is raised from a low-excitation energy orbit to a higher one.

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2GLSS P.H. Regan 18 Metallicity (CO p920) The iron (Fe, Z=26) content can be used as a good indicator of the age of the star. Newer stars have a higher iron content than their predecessors (more generations of reactions). The metallicity is defined as the iron-to-hydrogen (Fe:H) ratio in the atmosphere of a star compared to the solar value. This is given by the expression [Fe/H] = log 10 (N Fe /N H ) - log 10 (N Fe /N H ) o where log 10 (N Fe /N H ) o corresponds to the solar value. This stars with metallicities identical to the sun’s have [Fe/H]=0. Typical values range from -4.5 for old, metal-poor stars, to +1 for young, metal-rich ones. Note that some astronomers believe Type 1a supernovas may distort the local values of the [Fe/H] ratio for different regions of the inter-stellar medium (ISM) and thus prefer the [O/H] ratio instead as an measure of stellar age.

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2GLSS P.H. Regan 19 Forbidden Lines, ‘Metastable states, ‘isomers’ CO p404, Certain transitions from excited atomic and molecular states are hindered in their decay, usually by some quantum mechanical decay selection rule. This can give rise to very long lifetimes for such states. For such atomic states to exist in interstellar gas etc. the density must be very small since (as on earth) random atomic collisions would de- excite this state. Examples of such states at the  =21cm decay in the neutral hydrogen atom (H-I) and the green glow associated with some nebular arising from emissions in doubly ionised oxygen (O-III).

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2GLSS P.H. Regan 21 Hendrik Van de Hulst’s prediction of the observation of the 21 cm line allowed the study of cold, neutral hydrogen in the cosmos. If H atoms collide with neighbouring atoms, the atom can be raised from its anti-parallel spins ground state to the excited, parallel-spin config. This is a low-energy, excited metastable state (~10 7 years due to non-conservation of spin, 1s- >1s, but photon has intrinsic spin 1). Due to the low density, the atom can remain in this state for a long time before decaying back to the ground state via the 21 cm emission. This discovery was important because It’s a feature of H (most abundant element) It occurs only in low density regions It indicates the presence of neutral ( i.e.non- ionised and non-molecular) HI atoms It is not easily absorbed by interstellar gas. This means that 21cm radiation emission fromalmost anywhere in the galaxy can be measured on earth via radiotelescopes.

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2GLSS P.H. Regan 22 Hydrogen Molecules (H 2 ) Taken from RJS p369 (RJS p445) If the interstellar hydrogen is close to a hot star, the H 2 molecules can be ionised and form a region of H-II. Interstellar lines often show different component with slightly different wavelengths. This is caused by Doppler shifts which depend on the relative velocity of the specific cloud. Dust absorption means that emissions in the visible region are not useful in determining the overall structure of the galaxy.

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2GLSS P.H. Regan 24 The Interstellar Medium (ISM) Importance of radio-astronomy in seeing further due to less scattering/absorption in interstellar dust. Interstellar dust is heated to approx k by the stars in the galaxy, which then radiated in the far infra-red region (  m). Stars radiate strongly in the near infrared region (1-10  m). Very little stellar ‘extinction’ of light (as which occurs for the visible region) occurs in this range. See e.g., COBE spectra. Spin-flip transition in hydrogen gives rise to a 21cm radiowave emission. Thus, radiotelescope surveys allow the distribution if hydrogen across the plane of the milky way and its Doppler shift allows us to determine the speed at which the (H) gas in the galaxy rotates.

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2GLSS P.H. Regan 28 ISM, Composition (RJS p444) Interstellar Dust Early studies of UV radiation showed spectral features at 220 nm wavelength, corresponding to known transitions from graphite (carbon). Infra-red astronomy then showed that some stars were surrounded by dust shells which heat up and subsequently re-radiate in the infra-red region. Although these spectra are generally continuous, for some stars, an extra continuous peak was superimposed on the usual black body spectrum at wavelengths of approx. 10,000 nm. This was consistent with significant amounts of silicates (e.g. quartz SiO 2 ) in the dust cloud. It has been suggested that C and Si grains are formed in the carbon-rich atmospheres of giant pulsating stars. At expansion, the outer layers of such stars cool and the carbon atoms can stick together to make ‘grains’. When this region heats up again, the increased radiation pressure from the star pushes these out of the star’s atmosphere and into space.

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2GLSS P.H. Regan 29 Interstellar Gas (RJS p366ff,CO p447) The ISM also contains large amounts of gas, which is mostly made up of hydrogen. Depending on the density and temperature of the regions where this hydrogen is, it can exists either in its natural atomic form (HI), its stable molecular form (H 2 ) or the ionised form of the molecule (HII). In addition to hydrogen, a number of other molecules have been observed in the ISM, such as CO (carbon monoxide), SO 2 (sulphur dioxide), OH (hydroxyl), H 2 O (water), NH 3 (Ammonia), H 2 CO (formaldehyde), H 2 S (hydrogen sulphide) CH 3 CH 2 OH (ethyl alcohol!), HC 11 N (organic?). These molecules are identified by their emissions which occur in the UV, visible and IR regimes. Usually decays from excited states are by photon with wavelengths in the mm region corresponding from the decay from a rotational state to one with a slightly slower rotation. Such wavelengths can penetrate the earth’s atmosphere and be observed.

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2GLSS P.H. Regan 30 (CO p265) Dust particles scatter short wavelengths (blue) more effectively than long (red) ones. (Sky is blue, setting sun is red!) This effect can give rise to an interstellar reddening effect of stars which are partially obscured by dust clouds. This scattered light also tends to be (partially) polarised. Dust clouds can obscure the view of stars (and galaxies), and the centre of the Milky Way. The amount of interstellar extinction depends on the wavelength, , and the path length, s. Interstellar Reddening. Original light from source Interstellar dust cloud. Transmitted (red) light Scattered (blue) light

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2GLSS P.H. Regan 31 (See CO p265, p438) Absorption includes the scattering of light and true absorption from e.g., electrons being to higher energy states in atoms and molecules. It is wavelength dependent. If dI is the change in intensity through distance ds, then Opacity is the cross-section for absorbing photons of wavelength,, per gm of stellar material.

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2GLSS P.H. Regan 32 Spectral Maps of the Galaxy Ref

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2GLSS P.H. Regan 33 Standard Model of the Universe. Big Bang ~2x10 10 years ago, created an expanding universe, now 2x10 10 ly radius (constant expans.) Primordial Abundances: ~80% Hydrogen, ~20% Helium, trace amounts of Li and Be. After ~10 5 years, regions condense, gravitational energies leads to heating, nuclear reactions (proton-proton chain), stars form. H burns to He (p-p chain). For heavy stars, nuclear fusion reactions can burn to form elements all the way up to Iron (Fe, Z=26). Small stars eject planetary nebula which releases some material into space, but most kept in core (to form white dwarf). Large stars (>10M o ) have life cycles of ~10 7 years followed by cataclysmic supernova. Most of their material is expelled into the ISM. New stars form in the ISM which are metals- rich ( i.e., higher metalicities).

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2GLSS P.H. Regan 37 Distribution of Globular Clusters. (RJS p436) (1917) Before effects of dust were known, Harlow Shapley studied the distribution of globular clusters in space. He calculated their distances using (variable) standard candles known as RR- Lyrae stars located within these clusters. Shapley found that these clusters were further from the sun that thought and thus the galaxy must be larger than previously believed. Shapley reasoned that these large globular clusters were such large components of the galaxy that they would be unlikely to be distributed to one side. He thus proposed that the centre of the globular cluster distribution coincided with the centre of the galaxy, and thus that the sun was actually quite far from the centre. (Modern value is between 25-30Kly).

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2GLSS P.H. Regan 39 See also great pictures and info at the following.

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2GLSS P.H. Regan 40 Artist’s impression of above view of Milky Way from computer simulation. See web page given below. Milky Way from combined images at near-infrared wavelengths of 1.2, 2.2 and 3.4 microns from the COBE satellite. Note thin disk and central bulge. Redder regions are due to light absorption from dust.

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2GLSS P.H. Regan 41 Thin disk is metal-rich, [Fe/H]~0, star formation, lots of young blue stars. Around 325 parsec region of sun. Thick disk, metal-poor [Fe/H]~-0.5, older stars. Disk thickness increases towards the inner regions of galaxy.Gas/dust in disk absorbs visible light, but 21cm H-I line ok. Thus, radiotelescopes can map velocity and distribution of H-I gas. Galactic bulge. Spheroidal region near centre of galaxy. Only certain w.lengths observed due to dust. Disc meets the galactic bulge at a radius of approx. 1kpc. Vertical scale in bulge is around 400 parsecs (along the bulge’s minor axis). Major to minor axis ratio for the bulge is thought to be a ~0.6. Wide range in metallicity, in the bulge with - 1 < [Fe/H]

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2GLSS P.H. Regan 42 The surface brightness of the bulge, I, in units of L o pc -2 is given by the r 1/4 law, also known as de Vaucloulers profile (1948). Confirmed by COBE results. Re is the reference radius, I e is the surface brightness at r e. Formally, re is defined as the radius at which one half of the bulge’s light is emitted. Baade’s window is a gap in the dust clouds in the bulge which allows the observation of RR- Lyrae stars (standard candles) beyong the galactic centre. Appears 3.9 o below and within 550 parsecs of the galactic centre. Galactic B-field: Disk field estimated to be approx 0.4nT (~10 -5 times solar B-field). Deduced from Zeeman-splitting effect on the two states in H-I resposible for the 21cm line). Also from polarisation of scattered light.

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2GLSS P.H. Regan 43 Two distinct regions of metallicity for globular clusters, Generally, more metal-poor (older) in spherical distribution around galactic centre with more metal-rich (younger) in galactic plane. (CO p930) The Stellar Halo is the region around the disk and bulge. It is made up of a few hundred globular clusters and many high-velocity (‘field’) stars. The globular clusters consist of old, metal-poor stars, with the oldest clusters ([Fe/H]

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2GLSS P.H. Regan 44 Dynamics of the MilkyWay (CO p935, RJS p448) Motion of the Sun Note that the celestial equator ( i.e. the plane through the earth’s equator) is at 63 o to the galactic equator ( i.e., the plane through the galaxy’s disk). Definition of galactic co-ordinates, b (Galactic latitude) and l (Galactic longitude) are relative to the motion of the sun. NGP has co-ordinates of b=90 o. By convention Galactic centre has (almost) l=0 o and b=0 o. To north galactic pole (NGP) galactic centre star rotation l b sun

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2GLSS P.H. Regan 46 Local Standard of Rest (LSR) CO p942) To investigate the motion of the sun and other local stars, we must first define the Local Standard of Rest (LSR). This is defined as a point which is instantaneously centred on the sun and moving in a perfect circular orbits about the galactic centre. Thus, by definition, the velocity components about the LSR must be The velocity of star relative to the LSR is known as the peculiar velocity and is given by

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2GLSS P.H. Regan 47 The LSR has    km/s and a rotational period of about 230 million years. To a good approximation, there is no motion relative to the LSR in the R and z directions, but there is a significant  effect (see CO p943). The sun’s peculiar velocity (relative to the LSR) is called the solar motion and has values of: Motion of Sun in Galaxy (see CO p945)

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2GLSS P.H. Regan 48 Measuring Velocities (CO 107, p ) Proper motion. Change over time of stellar co-ordinates. Note that need to know distance (only useful for nearby stars). r vrvr vv observer star Doppler Shift Christian Doppler (1842) found that as the sound waves moved through air, the observed w.length, obs, is compressed in the forward direction and expanded in the backward direction (compared to a stationary observer) by the expression,

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2GLSS P.H. Regan 49 For light however, there is no speed of sound, as there is medium involved and the expression must be obtained using special relativity.  Source moving at velocity, u 2nd signal to observer 1st signal to observer u  If  t rest is the time difference between the emission of light crests and  t obs is the difference in time between their arrival at the observer then

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2GLSS P.H. Regan 50 Remembering that the frequencies of the light from the source ( rest ) and and the observed frequency ( obs ) are given by The Relativistic Doppler Shift is then given by For RADIAL MOTION ONLY Note v =vel. = freq

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2GLSS P.H. Regan 51 For objects moving away from the observer, there is a shift down in frequency ( i.e. up in wavelength) towards to RED end of the spectrum ( i.e. obs > rest and v r >0). For movement towards the direction of the observer, the shift is to the BLUE (i.e. obs < rest and v r <0) Since most astronomical objects are moving away from the earth, a redshift parameter, z, is used to describe the change in observed wavelength (and thus the radial velocity), where

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2GLSS P.H. Regan 52 Galaxy Rotation (CO p955-8, RJS p477) Can use the circular orbits of stars and Newton’s laws for objects far from the galaxy’s interior, equating the centripetal and grav. forces This predicted density dependence (~1/r 2 ) is much slower than deduced from the star counts beyond the centre of the galaxy. The density of luminous (i.e., visible) stars in the stellar halo appears to fall off as (~1/r 3.5 )….. This has been put forward as an argument that the majority of mass in the galaxy is actually in the form of Dark Matter. In a spherically symmetric system, the condition for mass conservation is given by (  =density)

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2GLSS P.H. Regan 53 Measuring Galactic Rotation Curves The rotation curves for galaxies are the measured velocities (via the Doppler shift) of H-I and carbon mononxide (CO) gases. The z- value observed in these lines can be used to calculate the radial velocity of the gas. Note that CO emits spectral lines which are easier detect through the region of the bright centre of a galaxy (unlike the visible H lines). //astrosun.tn.cornell.edu/courses/astro201/rotcurve.htm //www.owlnet.rice.edu/~spac250/elio/space.html //www.astro.ruhr_uni_bochum.de/geiers/GAL/gal_rot.htm Note, max Doppler shift (solid line above) is when motion is directly towards or away from observer.

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2GLSS P.H. Regan 54 (RJS p477) From Kepler’s 3 rd law as modified by Newton, for a star travelling about a galaxy with a Rotational Period, P in years, M star and M galaxy are in solar masses and R is the distance to the galaxy’s centre in AU, then This can be used to estimate mass of galaxies. CO p960)

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2GLSS P.H. Regan 56 Measuring the Doppler at each end of the galaxy gives the radial velocity as a function of distance from the galaxy centre This allows the rotation curve to be measured (Radial velocity, v r as a function of distance from galaxy centre, r)

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2GLSS P.H. Regan 58 The Galaxy (cont). n 21cm radiowaves from neutral H gas can penetrate the ISM. Large hydrogen clouds can be found in the spiral arms of galaxies (see e.g., M31). Looking across the galactic plane, one can observe a series of emission peaks, all slightly shifted with respect to each other. This can be explained by each spiral arm of the Milky Way moving with its own specific rotational velocity (and Doppler shift). It is thus possible to produce maps of the number and orientation of arms in the Milky Way. Other molecules (as well as H 2 ) such as OH, CH and CO can also be detected via radio frequency emissions. Giant Molecular Clouds (GMC), which represent stellar formation sites. CO which emits radiation at =2.6mm and regions of this molecule suggest that our galaxy has four major spiral arms (Perscus, Sagitarius, Centaurus and Cygnus). The Sun is situated on a smaller branched arm spiral known as Orion.

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2GLSS P.H. Regan 60 There is optical evidence for the spiral structure of the Milky Way, using so- called SPIRAL TRACERS (see RJS p203). These include the presences of young, (Population I) blue (O and B- type) stars, high-metallicity stars. Hot, ionised hydrogen (H-II) regions are also associated with stellar formation and might be expected to be found in the spiral arms. Other spiral tracers include (i) Emission Nebula illuminated by hot O and B stars, (ii) Giant Molecular Clouds (GMCs) and (iii) Supernova Remnants

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2GLSS P.H. Regan 62 Rotation Curve for the Milky Way (dark matter) n Optical and radioastronomy seem to indicate that all stars in the galaxy travel at the approx. the same rotational speed. Thus stars closer to the centre complete their orbits faster than the ones at the edge. This results in spiral shapes. n But as we know the approx. age of the galaxy and the rate at which the sun rotates around the galactic centre, the Milky Way should be more tightly wound than it is. n Failure of Kepler’s third law…more matter in middle maybe ? Dark matter ? n Why don’t the arms wind up ? n Supernovae may stretch new groups of stars into arms. Also spiral density waves and self-propogating star formation, the ‘ice-skater’ model. n Freeman’s Law (see CO p999, all spirals have ~equal surface brightnesses)

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2GLSS P.H. Regan 63 Supermassive Black Hole ? (CO p970, HOL p263) n Our galaxy is probably a ‘barred-spiral’. n The strongest infra-red emission line is found in a region at the centre of the galaxy called Sagittarius A (SgrA), which is also a strong source of synchrotron radiation (I.e. a point radio-source, Sgr A*. From around 1995 onwards, infra-red (2.2  m) enabled the velocities of ~90 stars close to to SgrA* to be measured. n Implication from Newtonian mechanics, radio-source and x-ray flares is a supermassive black hole with approx 2.6x10 6 solar masses at centre of Milky Way (see Ghez et al,. Nature 407 (2000) p349, see also n Distance of galactic centre is approx 26,000 ly from earth

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2GLSS P.H. Regan 65 Baganoff et al.,f rom the CHANDRA X-ray observatory, 01_releases/press_09050/flare.html False colour image of x-ray flare from supermassive black hole at centre of galaxy, associated with the compact radio source, Sagittarius A*. It is suggested that the flare is evidence for local matter falling into the black hole, which fuels energetic activity in the centre of the Milky Way.

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2GLSS P.H. Regan 66 Galaxies in General: A Brief History n Messier ( ) recorded 103 fuzzy objects (nebulae) in the Messier catalogue n Dreyer ( ) published the New General Catalogue (NGC) of almost 8,000 nebulae but not known at this point whether they were galactic or extra- galactic phenomena. n 1923, Hubble detected Cephied variables in M31 (Andromeda galaxy, NGC224), showed extra-galactic nature.

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2GLSS P.H. Regan 67 Morphology of Galaxies: Galaxy Types. (RJS p468, CO p990, HOL p257) Galaxies are broadly classified into three types by a scheme known as the Hubble Sequence. This divides galaxies into (i)Ellipticals (E), (ii) Spirals (S) & (iii) Irregulars (Ir). The spirals are further subdivided into two sequences, namely (iia) Normal Spirals (S or SA) and (iib) Barred Spirals (SB). There are also a class of galaxies which are transitional between ellipticals and spirals, known as Lenticulars, which can be either ‘normal’ (SO) or barred (SBO). These are arranged by Hubble’s Tuning Fork Diagram. ellipticals normal spirals barred spirals Hubble thought (wrongly!) this was an evolution of type

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2GLSS P.H. Regan 69 SPIRAL (Sa, Sb, etc.) galaxies consist of a large, dominant central bulge and tightly wound spiral arms. Star formation in the bulge region is thought to have taken place long ago and thus light from this region is blue-deficient (since massive, blue stars are relatively short lived), but rather reddish (from red-giants). This contrasts with the spiral arms, where new, blue, stars are formed. Spirals have (i) a flat disk, which usually contains a lot of interstellar matter and (ii) hot, massive, young (blue. O and B type) star clusters, which are usually arranged in spiral patterns structures. Some spirals have a large ‘bar’ through their central region and are known as ‘barred spirals’. (The milky way is thought to be a barred-spiral galaxy). Hubble subdivided spiral galaxies into Sa, Sab, Sb, Sbc, Sc and Sba, Sbab, SBb, SBbc and SBc. Those galaxies with the largest bulge- to-disk luminosity ratios, the most tightly wound spiral arms and the smoothest distribution of stars are classified as Sa or Sba. Conversely, Sc and SBc galaxies have loosely wound arms etc.

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2GLSS P.H. Regan 74 M82 Irregular galaxy IRREGULAR (Irr) galaxies have a chaotic appearance which does not resemble either spiral or elliptical galaxies. Many of the smallest galaxies are irregular (including the Magellanic clouds which accompany the Milky Way).

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2GLSS P.H. Regan 75 Size of Galaxies (CO p998) By mapping contours of constant brightness, (ISOPHOTES), one can infer the matter radius of galaxies. BUT, galaxies do not have have sharp edges, thus radii are not well defined. The effective radius (r e ), is defined as the radius within which half of the galaxies light is emitted. For the bulges of spirals and large ellipticals, this looks like (from equn, on page 42) where  ( r) is the surface brightness in units of magnitude/arcsec 2 and m e is the surface brightness at the effective radius, r e. (Note, that galaxial disks are usually modelled with an exponential, rather than r 1/4 decay).

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2GLSS P.H. Regan 76 The Tully-Fisher Relation (CO p1001, RJS p476) It is reasonable to assume that the greater the mass in a given galaxy, the larger its luminosity. It then follows, the greater the mass, the larger the gravitational force at each point in the galaxy would be. Since it is the size of this gravitation force which determines the rotation velocity of the galaxy (see equn. P52), there will be a relationship between the rotational velocity and luminosity of galaxies. The rotational velocity of galaxy can be deduced from the Doppler broadening of the 21 cm line. This will be both red and blue shifted. A value for the absolute luminosity for the galaxy can be estimated and compared with the observed brightness. The inverse square law of light can be used to make an estimate of galaxy’s distance. The relationship between the luminosity and rotational velocity of galaxies is known as the Tully-Fisher relation (1977).

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2GLSS P.H. Regan 77 Tully-Fisher Relations at large (0.2

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2GLSS P.H. Regan 78 Stellar Distributions and Metallicities H-I gas in our galaxy is distributed in spiral arms, connected to the bulge. Most young (blue) stars are in these arms, i.e., these are regions of on-going star formation. Since stars form from gas clouds, and massive (blue) stars form and die relatively quickly, one expects (all) massive, main sequence stars to be found in the spiral arms. Older stars are red in colour (further down main sequence, red giants..) These are found in the disk, not in the spiral arms. Red stars can are also found in the bulge and halo. Observations indicate that the metallicity of a galaxy correlates with its absolute magnitude and thus luminosity. This implies that chemical enrichment is more efficient in luminous, massive galaxies. Possibly due to them having more massive stars in which subsequently go to type II supernova ?

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2GLSS P.H. Regan 80 Galaxy Clusters and Colliding Galaxies (CO p , p1119ff) Most galaxies belong to clusters, which can contain 1000s of individual galaxies. Groups of galaxies usually have less than 50 members, with clusters containing between 50 and several thousand individual galaxies. Galaxy clusters are classified as either regular (spherical and centrally condensed) or irregular. The Milky Way belongs to the LOCAL GROUP, which consists of approx. 30 galaxies, including the Andromeda Galaxy (M31), M32, M33, M110 and the Small and Large Magellanic Clouds (LMC, SMC). The Milky Way and Andromeda galaxies are by far the largest and most dominant members of the local group. There are also around 20 small groups of galaxies within approx 14Mpc of the local group, including the Maffei-1, the South Polar or ‘Sculptor’, the M81 and the M83 groups. Out group of galaxies is in gravitational interaction with these galaxy groups.

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2GLSS P.H. Regan 81 The M81 Group is one of the closest galaxy groups to our own (~ 12Mly). The group has two large galaxies, M81 (‘Bodes galaxy’) and M82 plus NGC 3077 which all have a mutual gravitational interaction. Note the two dwarf galaxies close to M81 and the common gaseous envelope which are clearly apparent on on the radiowave image. visible radio M81 group M82 M81 NGC dwarf galaxies M81 M82 NGC3077 radio

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2GLSS P.H. Regan 82 It is thought that a few 10s of million years ago a ‘close encounter’ occurred between M81 and M82, with the result that M82 was dramatically deformed by the gravitational attraction of the larger and more massive M81. This encounter also left traces in the spiral pattern of M81, firstly making the overall pattern more pronounced. Note that the M81 and M82 are still relatively close at around 150kly apart. M81 M82

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2GLSS P.H. Regan 83 Gravitational attraction between nearby galaxies means they can interact and even collide with each other. Colliding galaxies does not imply colliding stars (size between individual stars too large). There can be dynamical friction between galaxies due to the gravity (see CO p1055). This dynamical friction force (‘drag’), f d, can be estimated assuming f d ~ (  / v 2 ) where v is the relative speed of the colliding galaxies and  is the density of the colliding material. Energy will also be converted from potential and self-energy into kinetic energy of individually scattered stars. Kinetic energy from the galaxies’ motions of can be transferred into internal KE of the gravitationally interacting galaxies. Gravitationally bound galaxies willeventually merge (e.g. Milky Way and LMC & SMC, also some evidence double nucleus in Andromeda, M31 galaxy). The mergers and tidal interactions ‘tidal stripping’ between colliding galaxies may be responsible for matter transfer and creating new active star-forming regions.

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2GLSS P.H. Regan 84 CO p1066

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2GLSS P.H. Regan 85

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2GLSS P.H. Regan 86 Calculating Galaxy Collisions (CO p1088) Realistic ‘N-body’ computer simulations can be carried out using Newton’s gravitational equations. Such calculations suggest that many large elliptical galaxies are actually the result of mergers between two spiral galaxies. It is thus thought that over time, spiral galaxies are destroyed by mergers, resulting in the creation of very large elliptical galaxies. Evidence for this includes Computer based N-body simulations The observation that elliptical galaxies appear to dominate in the more dense, centres of galaxy clusters Observations also suggest that the more distant (and thus younger) galaxies appear to have (had?) a greater fraction of spiral galaxies compared to ellipticals.

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2GLSS P.H. Regan 87 CO p1088

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2GLSS P.H. Regan 88 In hydrotstatic equilibrium, from the VIRIAL THEOREM (CO p56) the Total (E), Potential (U) and Kinetic (K) energies of a galaxies are related by the expression 2K= -U = -2E For a head-on galaxy collision, we can use the impulse approximation (CO p1061) to assume the encounter between the galaxies occurs so quickly that the individual stars do not have time to move much from their positions. The gravitational potential energy, U, of the galaxy is virtually unaltered by the collision. If a galaxy increases its internal kinetic energy, from K to K+  K as a result of the gravitational ‘work’ each galaxy has done on the other one, the total energy will also increase from K to K+  K, but the potential energy, U remains unchanged (=K). The galaxy would thus no longer be in hydrostatic equilibrium. To regain equilibrium, the extra 2  K of kinetic energy obtained just after collision, must be ‘lost’. One way this can be done is for this excess energy to be converted in to increased (i.e., less negative) gravitational potential energy, by expansion.

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2GLSS P.H. Regan 89 The Cartwheel galaxy (approx 500 Mly away), is an example of a ‘ring galaxy’ (see CO p1062) which followed from a nearly head-on collision between galaxies. This figure, from the Hubble Space Telescope, shows the effect of a collision by an intruder galaxy (it is not clear which one of the two objects to the right), which smashed through the core of the host galaxy. Note blue, new stars in the ring. The ring is expanding at approx, 90km/s. Colliding Galaxies

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2GLSS P.H. Regan 90 Sleeping Beauty Galaxy(M64) There is a central region where the stars rotate in a different direction to the dust and gas. This effect is thought to be result of collision between a large and small galaxy which has yet to equilibrate out.

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2GLSS P.H. Regan 91 Interacting galaxies Arp 273 ‘Interacting Galaxies, Arp 273’ distance approx 200Mly, false colour image from the Anglo-Australian telescope. Note the strong tidal distortion of the larger of the two galaxies and the notable ‘extra’ red nucleus in the smaller, side on galaxy in the botton right of the picture. The lower galaxy has a very active nucleus (see later).

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2GLSS P.H. Regan 92 Superclusters A supercluster is a group of galaxy clusters which appear to be associated (gravitationally) with each other. Unfortunately, this information is not usually known for most galaxy clusters and thus definitions of members of superclusters are made on the basis of how far different galaxy clusters are separated from each other. This definition has some problems, as the distances between galaxies is much more difficult to determine that the distance to them (since we can use the doppler shift to give us more accurate measures of the distance perpendicular to the radial direction, than the radial direction specifically.)

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2GLSS P.H. Regan 93 This extra gravity’ which is sufficient to hold the gas in place, has been proposed as further potential evidence for Dark Matter Perseus galaxy cluster, see False colour x-ray image The Perseus cluster of galaxies, approx. 300Mly. This is part of the Pisces-Perseus supercluster. Note that the confinement of gas in the large galaxial gravitational field (purple in figure). ROSAT Abell 426

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2GLSS P.H. Regan 95 The Virgo cluster of galaxies which is over 3Mpc across and consists of more than 2000 individual galaxies lies around 16 Mpc (~60 Mly) away. It represents the centre of the local supercluster which includes the local group. M87 M84 M86 ‘Makarian’s chain’ NGC4371 NGC4429 M89 M90 Central portion of the Virgo cluster

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2GLSS P.H. Regan 96 Expansion of the Universe, Hubble’s Law (CO p1110) Using luminosity of Cephied variable stars, Edwin Hubble plotted the recessional velocity of such standard candles in extra-galactic objects against their induced distance (deduced using the inverse square law of light). The resulting (nearly?) linear relationship is known as Hubble’s Law, which has the form, Where V is the recessional velocity and r is the distance from the earth. H is known as the Hubble constant.

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2GLSS P.H. Regan 97 From Hubble’s original paper, published in Proceedings of the National Academy of Sciences Volume 15 March 15, (1929), number 3. ‘Distances of extra-galactic nebulae depend ultimately on the application of absolute luminosity criteria to involved stars whose types can be recognised. These include….Cepheid variables, novae and blue stars involved in nebulae emission. ……The apparent luminosities of the brightest stars in such nebulae are thus criteria which, although rough and to be applied with caution, furnish reasonable estimates of the distances of extra-galactic systems’. ‘

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2GLSS P.H. Regan 98 Value of the Hubble Constant, Scale Factors and Hubble Time Hubble’s orginal value for H=530kms -1 Mpc -1. But subsequent measurements showed that he underestimated the distances involved as what he thought were the brightest stars were actually groups of stars. The value of H appears to have varied over time, thus while it is still a constant of proportionality between V and r, H=H(t). By convention, we the present day value of the Hubble constant is given the symbol, H 0. There is still much debate and measurement on the value of the Hubble constant. Currently, it is standard to define constant using the dimensionless quantity, h, and h has a value between 0.5 and 0.8.

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2GLSS P.H. Regan 99 Hubble’s law implies a universal expansion. In this context, the recessional velocity (due to the expansion of space) is distinct from the peculiar velocity (which is due to the motion through space). i.e v = H o d + v pec The recessional velocity arises because space itself is expanding, this is the realm of COSMOLOGY. The cosmological redshift needs to take into account the curvature of space-time (beyond the scope of this course). However, it is common practice to use the redshift equation to obtain the effective radial velocity. Thus, by measuring redshift, the distance can be determined. H 0 is poorly known, use H 0 =75(20)kms -1 Mpc -1 (h=0.75(0.2))